1. Field of the Invention
The present invention relates to a control valve for heavy construction equipment having a regeneration function.
2. Description of the Prior Art
"Regeneration" is to return a flow of fluid generated at return-side of an actuator to a supply-side of the actuator as a supplementary fluid in order to prevent a generation of a cavitation due to a shortage of fluid at the supply-side of the actuator and thereby achieving smoother operation.
Such a regeneration should be achieved utilizing the weight of an actuator associated with the regeneration, not a supply of fluid to the actuator obtained by a separate pumping means. For example, in an excavator, regeneration is to use a return flow of highly pressurized fluid, obtained upon lowering a boom by the weight of the boom, for raising the boom.
A typical conventional construction for achieving such a regeneration is illustrated in FIGS. 1 and 2. By referring to FIGS. 1 and 2, a control valve 100 is shown as having a main spool 101 in which a regeneration spool 103 is disposed to be resiliently biased at a predetermined pressure by a pressure setting spring 102. As the regeneration spool 103 moves, switching of fluid passages provided in the control valve 100 between opened and closed states is carried out to achieve regeneration. This will be described in more detail. As the main spool 101 moves in the direction indicated by arrow A in FIG. 1, a fluid passage 104 is opened to allow the interior of the main spool 101 to communicate with an actuator port C. A pressure generated by the return flow of fluid through the actuator port C serves to push the regeneration spool 103 in the direction indicated by arrow B in FIG. 1. As a result, a part of the return fluid flow discharged out of the actuator port C returns toward a tank T via a fluid passage 105. Meanwhile, the other part of the return fluid flows toward a pump P through a fluid passage 106 so that it is regenerated at the side of another actuator port opposite to the actuator port C, that is, the supply-side of the actuator.
However, such a conventional regeneration construction has a disadvantage of the impossibility to appropriately adjust the amount of fluid to be regenerated when an adjustment is required. This is because each of the fluid passages 104, 105 and 106 has only two switched states, namely, an opened state and a closed state. Here, the requirement to adjust the amount of fluid to be regenerated generally depends on the amount of fluid delivered from the pump. In other words, where the fluid amount delivered from the pump is sufficient, it is unnecessary to provide regeneration. If regeneration is obtained in this case, a flow of fluid delivered from the pump flows backward along the opened regeneration fluid line (the fluid passage 106 in the case of FIGS. 1 and 2) to the return-side of the actuator (the actuator port C in the case of FIGS. 1 and 2). This results in a loss of pressure at the supply-side of the actuator. On the other hand where the fluid amount delivered from the pump is insufficient, it is necessary to increase the amount of fluid to be regenerated in order to supplement the shortage of the fluid amount delivered from the pump.